The present invention relates to an improved single point mooring terminal or system (SPM) for ships, and is particularly concerned with a mooring system of the above type which is highly stable and efficient, and incorporates a spar buoy which is movable freely in a vertical direction, but which is restrained from lateral movement and from rotary movement by inclusion of a second, submerged buoy which restrains the spar buoy in an ocean current with minimum tilting, such buoy system also including a submerged housing which can be filled with air or other light fluid to provide an environment free of seawater for containing and operating motion compensators for piping, valves and the like, and permitting entry of personnel into such environment for inspection and maintenance purposes.
The single point mooring terminal presently employed consists of an anchored buoy which is used simultaneously for ship mooring and cargo transfer. The ship moors with bow lines, and is therefore free to swing around the mooring with changing weather conditions. This feature eliminates a major disadvantage of the traditional dockside and multi-point moorings which prevent ships from swinging while moored, and therefore required cargo operations to be carried out only in protected waters or favorable weather.
The single point mooring terminal is particularly advantageous for the loading and unloading of hazardous fluids such as crude oil and petroleum products, reactive and/or toxic chemicals, and refrigerated flammable gases such as liquid natural gas. The single point mooring terminal thus avoids possible collisions and disasters from major fire or explosion, by permitting the loading and unloading of these hazardous chemicals and fluids in remote waters away from congested waters near harbor entrances and within ports. Thus, hazardous cargoes can be transferred in isolated, restricted anchorages by means of the single point mooring terminal.
One principal disadvantage of presently employed single point mooring systems involves instability of the mooring system. Conventional single point mooring systems generally have a mooring swivel located on the top of the buoy above the ocean surface. Forces exerted by movement of the ship are therefore transmitted through the top of the buoy, causing various buoy motions which can limit use of the single point mooring system to favorable weather conditions.
Further, with most SPM systems presently employed, fluid cargo is transferred between the buoy and one or more seabed pipelines by means of flexible hoses, for transport to dockside facilities. A second set of flexible hoses runs along the water surface between the buoy and the ship's loading or unloading station. To accomodate swinging of the ship, multiple fluid swivel joint is located on the buoy, between the horizontal and vertical hoses. A principal disadvantage of such designs results from the location of flexible hoses, shutoff valves, and in some designs, the location of the piping swivel joint beneath the buoy. Safe and economic operation of the SPM requires reliable performance of these components over long periods of time. However, they function in a hostile environment, at a location where inspection and servicing are unusually difficult.
With refrigerated cargoes, such as cryogenic fluids, ice formation can also occur, leading to accelerated failure of flexible hoses. These components must therefore be insulated, leading to reduced flexibility and reduced operating life. When a cryogenic hose failure occurs or is imminent, replacement must be accomplished under the buoy without wetting internal surfaces. This accordingly is a difficult and complex operation. Further, except for the shutoff valves, these components are subjected to continuous movement by wave action and movement of the buoy itself. The use of conventional mooring lines attached directly to a single buoy allows significant lateral movement at low tide, due to slack in the lines, and this movement causes wear on the fluid connections between the buoy and seabed. This results in a relatively short useful life of the components and in addition the components can be degraded by corrosion or fouling in the marine environment.
Failure of a fluid component on a single point mooring system can cause environmental pollution, safety hazards, and loss of valuable commodities. Such failures also can result from fatique failure due to excessive movement of the mooring system, or from excessive strains due to fatique failure of the mooring system itself.
Another form of conventional SPM terminal consists of an anchored spar buoy. The conventional spar buoy becomes less stable when located in shallow water such as near the shore. With tidal changes, buoyancy varies and since the system is generally restrained near the extreme bottom, it is subject to tilting in a tidal current.
Various prior art systems have been developed in an effort to avoid many of the above noted problems.
Thus, U.S. Pat. Nos. 3,407,416; 3,474,749; 3,635,253; 3,894,567 and Published Patent Application No. B-379,955 disclose spar buoys, or slender, vertical cylinders weighted at the bottom for stability. All of these devices are moored using multiple chains running from the spar buoy to anchors on the seabed. With such an arrangement, the chains must be slack at low tide so that the buoy can rise with the tide. This slack means that the buoy has the disadvantage that it can drift laterally at low tide. In all of these patents catenary moorings are employed, that is the anchor chains are extremely heavy and therefore droop in catenary curves. The spar buoy must be large enough to support the weight of these chains with its buoyancy.
The devices of U.S. Pat. No. 3,614,869, and the above noted U.S. Pat. Nos. 3,407,416; 3,894,567 and Published Patent Application No. B-379,955 would appear to tilt excessively in areas of strong ocean currents. This is due to the fact that the mooring chains are attached well below the buoy's center of drag, and/or the ship is attached to the buoy well above the attachment point for the anchor chains. In either case, a strong ocean current would cause excessive tilting of the buoy. These patents, and also U.S. Pat. No. 3,837,380 employ either flexible hoses, pipe with swivel joints, or pipe with ball socket joints immersed directly in seawater. They are therefore difficult to inspect or maintain, subject to corrosion at critical points, and would be non-feasible for transfering a low temperature cryogenic fluid such as liquid natural gas, due to icing of the otherwise movable parts. It is noted that U.S. Pat. No. 3,837,380 mentions the use of a natural gas liquefier on the moored ship to avoid this problem. However, natural gas is customarily liquefied at a shore facility and piped to the ship as cold liquid, and relocation of the liquefier on the ship itself is a costly and undesirable change.
U.S. Pat. No. 3,474,749 involves a spar buoy with a concentric, buoyant collar. However, in this case the collar is the surface platform, or primary buoy, while the vertical cylinder is used only to increase tension in the catenary mooring chains. The patent does not disclose fluid transfer conduits or ship mooring capability.
U.S. Pat. No. 3,407,406 describes a system with long, vertical cylinders which can be flooded to vary buoyancy of the system. However, this capability is employed only to change the spar buoy's orientation from horizontal to vertical during installation.
U.S. Pat. Nos. 3,155,069; 3,404,654 and 3,675,609 disclose systems for controlling relative motion between the surface platform and the moored ship, and do not relate to problems of buoy stability of fluid transfer between the seabed and the ocean surface. Although the latter two patents disclose a skirt open at the bottom, which entraps a fluid of lower density than seawater, the fluid so trapped is oil employed to lubricate a mechanism within the skirt.
U.S. Pat. No. 3,778,854 discloses a mooring and oil transfer system including an elongated vertically positioned spar buoy wherein the ship's mooring line is attached near the top of the spar buoy above the water line, and to which is also attached a large crane. This construction causes the buoy to tilt, and variable ballast is provided to offset such tilting. An anchoring collar is rigidly attached to the spar buoy as a means for connecting the anchoring chains. However, as the relative positions of the spar buoy and collar cannot be altered with changing tidal conditions, the system is restrained in an optimum manner only at certain times during each tide.
A particular object of the present invention is the development of a reliable single point mooring system to minimize movement due to tidal and wave effects. Another object is the provision of means in such stable single point mooring system which permits housing of motion compensators for piping in an air atmosphere rather than seawater to thereby permit transfer of low temperature cryogenic fluid such as liquid natural gas, without icing of the surrounding seawater, and which permits access to such zone by personnel for inspection, maintenance and repair operations. Still another object is the provision of means for mooring a ship to the buoy so that the ship's presence has a negligible affect on motion characteristics of the buoy system.